Abstract: The driver bit according to the present invention includes: a tip on which a cross blade is formed; a torque transmitting part having a polygonal outer circumference; and a locking groove which extends in the circumferential direction between the tip and the torque transmitting part and into which locking balls enter, wherein the tip has four blade forming grooves forming the cross blade, the blade forming grooves are continuous with the locking groove, and the blade forming grooves are arranged so that the blade forming grooves have bottom parts, at which they are connected with the locking groove, located inwardly of the outer circumference of the torque transmitting part, and the center lines are displaced from a line connecting the shaft center with vertex positions of the polygonal outer circumference and a line connecting the shaft center with center positions of sides of the polygonal outer circumference.

Abstract: Provided is a bit holder device which is capable of driving a screw into a workpiece, while holding the screw having a groove in which a bit body is inserted, and which makes it possible to change the bit body without removing it from a chuck of an electric tool, etc. A bit holder device facilitating the attachment and detachment of a cylindrical body is also provided. A bit holder device for driving a screw, while holding the screw against detachment from a bit body with retaining a state where an end of the bit body is inserted in a groove on a head of the screw, has a cylindrical cover member removably fitted about the bit body from the end of the bit body, the cover member has an end including a magnetic attracting portion for magnetically attracting the head of the screw, and the bit body protrudes from the end of the cover member.